Amelia Virginia Hesketh, Cassandra Konecny, Sandra Emry, Christopher D. G. Harley

To accurately predict how organisms and ecological communities will respond to future conditions caused by climate change, we must consider the temporal dimension of environmental stressors, including the effects of repeated exposures to stress. We performed a two-year passive warming experiment in coastal British Columbia to explore the response of intertidal communities to single and successive warm summers. Elevated summertime temperatures tended to reduce the abundance of barnacles and grazers, algal cover, and alpha diversity compared to ambient temperatures, and both contemporaneous and persistent effects of warming were detected. While elevated summer temperatures appeared to have direct effects on organism survival and growth, the persistent effects of warming through time and differences in community structure between treatments were likely mediated by differences in foundation species (barnacle) abundance between treatments. Unexpectedly, the effects of thermal stress in year two were rarely dependent on whether there had been thermal stress in year one. Our study suggests that, while barnacle beds can recover from single warm summers, recurring thermal stress will result in a more depauperate, less diverse community over time, particularly if foundation species are negatively affected.